This invention relates to communication links, and more particularly to a two-way data link using communications satellites to provide low rate data communication with aircraft.
There has been a long-standing interest in the civil aviation community in ways to improve air/ground communications over ocean areas. Communications satellites offer a potential to satisfy this need. However, previous efforts to provide voice and medium-speed data service to civil aviation on the basis of a dedicated satellite system have not come to fruition primarily because of cost.
It has been estimated that the average data rate per aircraft required for oceanic air traffic control (ATC) is on the order of 4 bits per second or less. For a number of reasons, data rates this low may not be practical. Generally, the use of data rates below 1200 bits per second with the corresponding low power and low carrier-to-noise density (C/N.sub.0) levels is subject to two major limitations. First of all, carrier frequency acquisition using conventional methods requires much more search time as the C/N.sub.0 decreases. This increased search time reduces the effectiveness of the system by requiring long carrier preambles in which no information is transmitted. The carrier acquisition problem is exacerbated in aeronautical applications where the initial frequency uncertainty is high because of the possibility of high Doppler shifts due to aircraft motion. Secondly, as the data rate and C/N.sub.0 are lowered, system phase noise, most of which is due to microwave frequency sources on a communications satellite, becomes an increasingly significant source of performance degradation.
It is therefore an object of the present invention to provide a low data rate communications link which eliminates the need for long carrier preambles and reduces system phase noise.
Another object of the invention is a low data rate communications link having lower power and antenna gain requirements.
Yet another object of the invention is such a communications link which provides economically attractive communications over ocean areas.
Still another object of the invention is a communications link in which communication satellites and earth stations are shared between aeronautical users and other mobile users.